1. Field of the invention
The present invention relates to a rack assist type power steering apparatus, and more specifically, to a rack assist type power steering apparatus in which: a central portion of an external-screw-groove-formed part of a rack bar, which slides leftward and rightward according to a driver's steering, can reduce a friction generated between balls and an external screw groove, thereby reducing noise and effectively transmitting the power of a motor to the rack bar; and opposite end portions of the external-screw-groove-formed part of the rack bar can reduce the loads that are applied to the balls by virtue of a vertical load to prevent damage to the balls and improve the durability of the balls, thereby reducing the loads applied to the balls while decreasing the friction with the balls in the external-screw-groove-formed part of the rack bar.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of a rack assist type power steering apparatus in the related art. FIG. 2 is a schematic sectional view illustrating a part of the rack assist type power steering apparatus in the related art.
As illustrated in the drawings, the rack assist type power steering apparatus in the related art includes: a driving unit 120 that is controlled by an electronic control unit (ECU) to generate power, the electronic control unit (ECU) determining the degree to which a steering wheel 113 is turned through a torque sensor 111 that is mounted on the upper end of a steering column 110; a rack bar 130 that moves tie rods 133, which are connected to wheels 131, according to the degree to which the steering wheel 113 is turned; and a driven unit 140 that receives the rotating force of the driving unit 120, converts the received rotating force into an axial moving force, and transmits the axial moving force to the rack bar 130.
Here, the driving unit 120 includes: an electric motor 121 that is controlled by the electronic control unit (ECU); a driving pulley 123 secured to the shaft of the electric motor 121; and a driving belt 125 wound around the driving pulley 123.
Further, the driven unit 140 includes: a ball nut 141 that is provided within a rack housing, which surrounds the rack bar 130, and supports the rack bar 130; and a driven pulley 143 coupled to the outer circumferential surface of the ball nut 141.
The driving pulley 123 connected to the shaft of the electric motor 121 and the driven pulley 143 coupled to the ball nut 141 are arranged parallel to each other, and the driving belt 125 is coupled to the driving pulley 123 and the driven pulley 143 to transmit the rotating force of the electric motor 121 to the rack bar 130. The rack bar 130 is moved leftward and rightward by the operation of the ball nut 141 to generate an assist force.
An internal helical screw groove 201 that has a Gothic arc shaped cross-section is formed in the inner circumferential surface of the ball nut 141, and an external helical screw groove 203 that has a Gothic arc shaped cross-section is formed in the outer circumferential surface of the rack bar 130. Balls 210 are inserted between the internal screw groove 201 and the external screw groove 203. The balls 210 move by rolling while making contact with the internal screw groove 201 and the external screw groove 203, and circulate through an intermediate passage 205 that is formed by the ball nut 141 and end caps 220 coupled to opposite inner sides of the ball nut 141, respectively.
Meanwhile, a central portion of the external-screw-groove-formed part of the rack bar, which slides leftward and rightward according to a driver's steering, frequently contacts the balls, which increases friction, causes noise, and precludes the power of the motor from being effectively transmitted to the rack bar.
Further, when a driver steers to the maximum, opposite ends of the external-screw-groove-formed part of the rack bar move into the ball nut, and the varying angles of the tie rods increase accordingly so that the loads applied to the opposite ends of the rack bar become larger. Due to this, the loads applied to the balls increase, thereby causing scratches on the balls and deteriorating the durability of the balls.